Computer/Embedded Technology


Putting Flash lifespan into perspective

21 September 2005 Computer/Embedded Technology

Most industrial PCs or single board computers (SBC) are fitted with CompactFlash sockets. This allows the system to boot and run from a CompactFlash instead of a hard disk drive. Compact Flashes are physically smaller and can handle shock and vibration much better than hard disk drives. They have simulated heads, sectors and cylinders just like normal hard disk drives, allowing the PC BIOS to treat them as normal hard disk drives.

Unfortunately, Flash devices have a limited number of write cycles, which often results in the non-selection of Flash as a data storage medium. Low-cost 'retail grade' compact Flashes can be specified to have as low as 100 000 write cycles. 'Industrial Grade' CompactFlash can be specified to have up to 3 million write cycles. In many cases, this might still seem low. Note that one write cycle means writing to the whole Flash once. If you write 100 KB at a time to a 100 MB Compact Flash, you would need to write this amount 1000 times to constitute one write cycle. Flash has wear levelling built into the design.

Example: Let us assume you have an Industrial Grade 1 GB CompactFlash of which the operating system takes up 250 MB (XPembedded, for example) and the remaining 750 MB is used for data storage. Now, you can write data blocks of 100 KB 7500 times to make up one write cycle (750 MB/0,1 MB). This means that you can do 3 million times 7500 writes of 100 KB before the Flash is used up. If you write once every 5 seconds (24/7) it will last 3E6 x 7500 x 5 seconds or 3567 years which is surely long enough (this is based on theory - actual result may vary a lot but even if one divides the time by 100, the period is still long enough for most applications.)

Flash lifespan can be increased further if you use an operating system such as XPembedded and select the EWF (enhanced write filter) option. This filters out writes to the Flash and redirects the written data to other locations such as system RAM. From time-to-time, the data can be copied from RAM onto the Flash, resulting in a huge saving in the number of writes to Flash. A noteworthy point, is that 'Flash lifespan' is normally not the first reason why a system fails.



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